The present invention relates to a method and apparatus of detecting faults for a fuel evaporative emission treatment system and, more particularly to a method of precisely detecting the airtightness of a fuel tank.
In general, automobiles emit harmful substances such as carbon monoxide, nitrogen oxides, and hydrocarbon. For example, unburned hydrocarbon (HC) gas contained in blowby gas or exhaust gas is emitted to the atmosphere as HC, and crude gasoline (fuel evaporative emission) evaporating in a fuel tank or the like is dissipated into the atmosphere. Therefore, automobiles are equipped with a device for controlling or suppressing the emission of harmful substances, such as an exhaust gas purification device or a fuel evaporative emission treatment system.
The fuel evaporative emission treatment system, which prevents the dissipation of fuel evaporative emission into the atmosphere, is typically provided with a canister having activated charcoal for adsorbing HC. The canister has an inlet port communicating with the fuel tank, an outlet port communicating with the suction pipe of engine, and a vent port which is open to the atmosphere. In the canister storage type fuel evaporative emission treatment system of this kind, fuel evaporative emission (HC) in the fuel tank is admitted into the canister when engine is not in operation, and adsorbed by activated charcoal in the canister. As the engine is run subsequently, a negative pressure of the suction air produced in the suction pipe acts on the outlet port to admit purge air through the vent port, so that HC absorbed by the activated charcoal is separated from the activated charcoal by the purge air, and the separated HC is discharged to the suction pipe together with the purge air. The HC (fuel evaporative emission) discharged into the suction pipe burns together with the air-fuel mixture in the engine cylinder, thereby preventing the dissipation of fuel evaporative emission into the atmosphere.
The canister storage type treatment system is classified into two types: One is a manifold port purge type in which a small hole for admitting fuel evaporative emission into the suction pipe is formed in the suction pipe on the downstream side from the throttle valve. The other is a throttle port purge type in which the small hole is formed in the suction pipe at a position such that the small hole is located on the downstream side from the throttle valve when the throttle valve is opened by a predetermined degree of opening or more from the fully closed position.
The fuel tank system consisting of a fuel tank, pipes, hoses and the like sometimes becomes incompletely airtight. For example, the airtightness around the fuel cap may be incomplete, or a small hole may be formed in the fuel tank body. If the fuel tank system is incompletely airtight in this manner, fuel evaporative emission dissipates into the atmosphere. In particular, if fuel evaporative emission cannot be admitted into the canister from the fuel tank due to the clogging of the purge passage connecting the inlet port of canister to the fuel tank caused for any reason, fuel evaporative emission becomes liable to be dissipated via a non-airtight (leak) portion of the fuel tank system.
If fuel evaporative emission cannot be discharged to the suction pipe from the canister due to the clogging of the purge passage connecting the outlet port of canister to the suction pipe, fuel evaporative emission is admitted into the canister from the fuel tank exceeding the HC adsorption limit of activated charcoal. In this case, fuel evaporative emission is dissipated into the atmosphere from the vent port while the vent port of canister is open.
Even if fuel evaporative emission is dissipated into the atmosphere in such a manner, the operation of engine is not affected. Therefore, the driver does not perceive this abnormality, so that he/she leaves the abnormal condition as it is, thereby fuel evaporative emission continuing to be dissipated into the atmosphere.
To solve the above problem, systems and methods of detecting the abnormality of the fuel evaporative emission treatment system have been proposed. Typically, an alarm is given when the abnormality of the treatment system is detected, and the driver takes a proper measure in accordance with this alarm, thereby the dissipation of fuel evaporative emission into the atmosphere being inhibited.
For example, Japanese Patent Publication No. 505491/1992 corresponding to International Publication No. W0091/12426 discloses an automotive tank venting device and a method of inspecting its proper function. This device is provided with an adsorption filter connected to the fuel tank via a filter pipe, and a valve pipe connecting the adsorption filter to the suction pipe of internal combustion engine. The vent pipe of adsorption filter has a shutoff valve, and the valve pipe has a tank vent valve. The above-mentioned inspection method comprises a step in which the tank vent valve is opened with the vent pipe being shut off, and a step in which whether a negative pressure is produced in the fuel tank or not is determined. If the difference between the atmospheric pressure and the internal pressure of the fuel tank exceeds a predetermined threshold, and therefore a negative pressure is produced in the fuel tank, it is judged that the device functions normally. That is to say, if a negative pressure is produced in the fuel tank, it is judged that the filter pipe and the valve pipe (corresponding to the aforesaid purge passage) are not clogged and that the tank vent valve or the device is airtight. If a negative pressure is not produced in the fuel tank, fault information is sent.
With the method disclosed in Japanese Patent Publication No. 505491/1992, the airtightness of the fuel tank system including a fuel tank, a filter pipe (purge passage), a tank vent valve (purge control valve) and the like can be determined to a considerable degree. Specifically, when the airtightness of the fuel tank system decreases to a degree such that the internal pressure of the tank exceeds the threshold Just after a negative pressure is introduced, poor airtightness can be detected. If the degree of poor airtightness is small, the internal pressure of the tank does not exceed the threshold by the time when the airtightness is determined; therefore, poor airtightness is not detected. Even if the airtightness is slightly poor, fuel evaporative emission is dissipated into the atmosphere.
The value of negative pressure of suction air produced in the suction pipe of engine, and in turn the value of a negative pressure produced in the fuel tank when the introduction of negative pressure is completed vary depending on the degree of airtightness of the fuel tank system and the operating condition of the engine. It is therefore actually difficult to set the threshold in such a manner that the airtightness can be determined precisely in various tank airtightness conditions and engine operating conditions. In particular, if the threshold is set in such a manner that slightly poor airtightness can be detected, the airtightness is sometimes judged to be poor despite the fact that the airtightness is actually good, depending on the engine operating condition at the time when the airtightness is judged.
To introduce a negative pressure for determining the airtightness, the vent pipe must be shut off (the vent port of canister must be closed) as described above. Therefore, as a negative pressure is introduced, fuel evaporative emission is sucked into the suction pipe. In other words, the air-fuel mixture supplied to the engine when a negative pressure is introduced in the fuel tank is enriched excessively by the effect of the fuel evaporative emission supplied into the suction pipe from the fuel tank. If such an excessively rich mixture is supplied to the engine operated in an operation range in which the amount of suction air is small, there occurs unstable combustion, which causes fluctuation in engine output torque, and other problems.